Our previous article about Smart IMS (Smart Energy International 4/2002) was devoted to the evolution of our electric energy meter. This time we want to discuss communication – the cornerstone of the AMR concept. What communication methods exist for automatic meter reading? Why did we prefer PLC technology? What are the characteristic features of our approach to communication using an AMR system? In this article we try to answer these questions.

When building an AMR system, particularly for the residential sector, two communication technologies are used more frequently than others. Either radio frequency (RF) with the use of specialised receivers/transmitters, or data transmission via the existing power lines (PLC) with the use of special modems, is chosen as the optimum method to communicate with a meter.

THE BENEFITS OF PLC

In Smart IMS PLC technology is used, because in our opinion it has several advantages over RF.

• Cheap RF-solutions – that is, solutions based on low-powered transmitters and not requiring a licence from the Federal Communications Commission (FCC) – have limited range, about 30/100 metres for apartments and individual houses respectively. Even when the data collection problem is successfully solved at the local level, this limitation leaves the problem of transmitting the data to the remote data collection and management centre.

It is for this reason that some AMR system producers offer half-automatic technologies of data collection, by the use of a special hand terminal. Such technologies do not provide optimum metering data reception at the collection centre, neither do they allow additional meter management functions from the centre. Thus cheap RF solutions cannot serve as a base to build a complete AMR system which meets all modern requirements.

• More expensive RF solutions make complete AMR systems available, but create a sequence of other problems for the potential user. First of all, it is necessary to solve the problem of radio-frequency licencing which is more difficult in some countries than in others. Secondly there is the question of installation of receivers-repeaters, and of protecting them from possible vandalism. Thirdly one has to consider the relatively high cost of the equipment to be used.

PLC technology is devoid of these defects. The cost is quite acceptable; PLC allows a two-way data exchange with the meters via power line (0.4 kV); licencing and the construction of additional communication channels (lines) are not required. Indeed, all the possible installation spots for communication equipment are neatly defined – LV/MV transformers with a special access mode.

Many manufacturers are moving towards the use of power lines as the preferred communication environment. However there are at least four important aspects which should also be taken into account when choosing PLC.

THE METER AS AN INTEGRATED SOLUTION

In ADD GRUP we realised a long time ago that using any external modules (terminals) to connect meters to the communication system had no future. Instead we offer an effective integrated solution – a meter with a built-in PL modem. The result is broadening of the meter's functionality, reduction of the total equipment cost and an increase in the ability to protect the communications components of the meter. Not all manufacturers of AMR systems are able to offer this; it is only available from those who are meter manufacturers as well.

Typical scheme of data collection with the additional repeater

USING THE METER AS A REPEATER

Figure 1 shows the typical method of data collection for the majority of AMR systems based on PLC. Direct data exchange between the meter and the concentrator is not always possible, because of the large distances between the devices, the presence of disturbance on some network segments and the peculiarities of the electrical network's configuration. Traditionally attempts have been made to solve this problem by adding additional devices in the form of repeaters. In our opinion, such an approach is ineffective, for the following reasons:

• The introduction of additional devices raises the total cost of the system.

• There are often problems in choosing the best place to install the repeaters.

• Changing the configuration of the electric system means that the repeaters often have to be reinstalled.

We have chosen another way to solve the problem – re-equipping the meter with a retransmission function. In this way each meter is capable of assisting another meter when the last one is contacting the concentrator. This means that every meter can support up to seven levels of retransmission. (See figure 2).

Using meters as repeaters

This system also allows the meter to choose an optimal path between it and the concentrator for every communication session – the path which has the minimum number of retransmission levels and acceptable quality of transmission. As a result we have a system where there are no additional devices; data transmission is supported at any length and configuration of the system; and there are no problems in the event of a change of network configuration in the future.

DATA RATE

At the earliest stage of AMR, data rates were not considered a critical parameter. System developers focused on the provision of a transmission facility, together with an acceptable level of reliability. It seemed that low speed transmission (for example, 100 bps) on one frequency would satisfy system users, as meter readings only took place once a month.

At the earliest stage of AMR, data rates were not considered a critical parameter. System developers focused on the provision of a transmission facility, together with an acceptable level of reliability. It seemed that low speed transmission (for example, 100 bps) on one frequency would satisfy system users, as meter readings only took place once a month.

However, modern trends in the electric power distribution market have introduced new requirements for power engineers to solve – multiple-tariff support, effective reception of big data files from intelligent meters, the need for control and even power consumption management. Communication channels with a higher throughput are needed to meet these requirements.

We estimate that the minimum data rate level required today is 1200 bps, to accommodate modern requirements. As a result, ADD GRUP has developed and uses PL modems at 1200 and 2400 bps in the structure of Smart IMS. We are also planning to release a modem at 4800 bps. In addition our modems are able to carry out transmission/reception functions on several frequencies, enabling the system to adapt to changing channel conditions.

COMPLEX APPROACH TO THE COMMUNICATION ENVIRONMENT

ADD GRUP has complex solutions available to users of AMR systems – technologies and devices that enable information support inside the whole system, from the meter to the data processing centre, which may be located many kilometres away. Users are given a wide choice of the most appropriate communication medium between the substation and the centre (GSM, CDMA, PL MV and TL). Our system offers flexibility and reliability to utilities planning to introduce automatic meter reading.